1. Field of the Invention
This invention relates generally to the fabrication of semiconductor devices and circuits in which a sulfide layer is formed on the surface of a selected substrate and, more particularly, to the deposition of a sulfide layer or the formation of a native sulfide layer on a substrate at a low temperature and without causing charge damage or radiation damage to the substrate.
2. Description of the Prior Art
In the manufacture of semiconductor devices and circuits, it is often necessary to form an insulator or dielectric layer on the surface of a semiconductor substrate to provide electrical insulation between adjacent layers or structures. The physical and electrical properties of the dielectric layer are important in determining the electrical performance of the completed device or circuit. Frequently used dielectric materials include silicon dioxide, glasses of silicon dioxide and other oxides, silicon nitride, aluminum oxide, and organic films, such as polyimide or Teflon, as discussed, for example, by J. A. Amick, G. L. Schnable, and J. L. Vossen, in the publication entitled "Deposition techniques for dielectric films on semiconductor devices", in the Journal of Vacuum Science and Technology, Vol. 14, No. 5, Sept./Oct. 1977, at page 1053. In addition to these latter materials, sulfides have more recently been used to provide thin film passivation layers which are transmissive to radiation in the infrared range, for use in such devices as infrared radiation detectors.
The conventional methods for forming sulfide thin films involve either sputtering or evaporation processes. In accordance with a known non-reactive sputtering process, as described, for example, by B. R. Critchley and P. R. C. Stevens, in the Journal of Physics D, Applied Physics, Vol. 11, 1978, pages 491 to 498, a disk of a selected sulfide material, such as zinc sulfide (ZnS), is bombarded in a reaction chamber with inert ions, such as argon ions. The bombarding inert ions cause the ZnS to vaporize from the target (disk), and the vaporized ZnS subsequently deposits on the selected substrate. In such a sputtering process, the bombarding ions are formed by subjecting the chosen bombarding material, such as argon, to a radio frequency (rf) or direct current (dc) discharge. However, as a result of the exposure of the chosen bombarding material to the discharge, numerous extraneous ionized and neutral particles and high energy radiation with wavelengths as low as 500 angstroms or lower are produced. These extraneous particles then bombard the surface of the substrate on which the sulfide is being formed and cause damage thereto by altering the quantity and distribution of charge therein. In addition, the bombardment of the substrate surface by these particles causes the formation of additional charged particles and radiation, which may also damage the substrate. This alteration in the charge of the substrate undesirably alters the electrical performance of the substrate and any structures formed therein. In addition, the deposited sulfide may incorporate charges or dangling bonds, which create high surface state densities at the interface between the semiconductor substrate and the deposited sulfide, and which will trap charges when a voltage is applied to the device, thereby preventing optimum device performance. The damage produced by charge bombardment and radiation bombardment is particularly noticeable when the substrate comprises an electrically sensitive device, such as a charge coupled device or a device formed of certain compound semiconductor materials, such as mercury cadmium telluride, indium antimonide, or gallium arsenide.
In accordance with a known evaporation process to form a thin sulfide layer, as described, for example, by K. Pulker and J. Maser, in Thin Solid Films, Vol. 59, 1979, pages 65 to 76, a source comprising the selected sulfide, such as arsenic sulfide, is placed in a reaction chamber and is raised to an elevated temperature sufficient to cause evaporation of the sulfide, which subsequently deposits on the selected substrate. However, the sulfide films formed by evaporation processes generally have non-uniform surface morphology and non-reproducible deviations from stoichiometric composition, which degrade the electrical performance and reliability of the device on which the sulfide layer is formed.
It is the alleviation of the prior art problem of imparting damage to sensitive devices due to charge bombardment and radiation bombardment during the formation of a good quality sulfide layer thereon to which the present invention is directed.